292 E. M. XELSOX ON METHODS OF ILLUMINATION. 



daylight illumination we must substitute image of window for 

 image of lamp-flame. This image will vary according to circum- 

 stances. It may be, for instance, a gap between chimney-pots, or 

 perhaps between houses; in my own case it is an irregular 

 trapezium, formed by the top and one side of a window and a 

 yew-tree. It is only a privileged few who have an unlimited 

 area of unbroken sky, coupled with the idealistic and immovable 

 white cloud. It is my unfortunate experience to find that white 

 clouds always go just at the precise moment a delicate point is 

 being caught, and then appears that terrible blue sky, so eagerly 

 sought by many, but which drives a microscopist to distraction. 

 It is remarkable that these fundamental principles of elementary 

 microscopical manipulation have never been explained in any 

 text-book on the subject. 



The Ramsden disc is an image of the back lens of the objective. 

 If the Kamsden disc be too small for examination by the unaided 

 eye, the eyepiece may be removed and the back lens itself 

 examined ; but probably the simplest and quickest method is 

 to employ a " loup," for it saves the trouble of removing and 

 replacing the eyepiece. 



Ground Glass. Before dismissing this subject there is yet 

 another kind of illumination to be dealt with, namely, that by 

 ground glass. This illumination was largely used by Dr. Car- 

 penter, and is much advocated by Lewis Wright in his admirable 

 book on the microscope. I became first acquainted with ground 

 glass in 1875, by purchasing a Swift's excellent XJniv^ersal Con- 

 denser, which he had brought out the previous year. This 

 condenser (an improvement on Hall's, made by Swift in 1868) I 

 still have, and use.* The top lens is removable ; the back, consist- 



* So far as I am aware, these condensers were the first for low powers 

 ever constructed, and the microscopical world is greatly indebted to the 

 late James Swift, not only for them, but also for many excellent improve- 

 ments, both in the bra!>s and glass of the microscope. 



This particular condenser is, I understand, no longer made ; therefore a 

 fuller description, showing wherein it differs from its modern substitute, is 

 necessary. This condenser has an uncorrected front lens, and a pair of 

 achromatised doublets at the back ; therefore, when used as a whole, it is 



